1. Field of the Invention
The present invention relates to an exhaust emission purification device for an internal combustion engine.
2. Description of the Related Art
Generally, in the case where an exhaust gas flowing into an exhaust emission purifier, carrying a NOx holding agent, contains a sulfur component (SOx, etc.), the exhaust emission purification capacity of the exhaust emission purifier is known to be deteriorated by sulfur poisoning.
In order to prevent the exhaust emission purification capacity of the exhaust emission purifier from being deteriorated due to sulfur poisoning, Japanese Unexamined Patent Publication No. 6-346768 discloses an exhaust emission purification device in which a sulfur component holding agent capable of holding the sulfur component in the exhaust gas flowing thereinto is arranged upstream of the exhaust emission purifier in the exhaust gas. In this exhaust emission purification device, when the sulfur component held by the sulfur component holding agent is released, the exhaust gas containing the released sulfur component is prevented from flowing into the exhaust emission purifier thereby to protect the exhaust emission purifier against the sulfur poisoning. As described above, an exhaust emission purification device having an exhaust emission purifier is required to avoid sulfur poisoning of the exhaust emission purifier.
With an exhaust emission purifier carrying a NOx holding agent, the air-fuel ratio of the exhaust gas flowing into the exhaust emission purifier is enriched by a rich spike in order to release the NOx held in the NOx holding agent. When introducing the rich spike, i.e. when enriching the air-fuel ratio of the exhaust emission gas, fuel is required. The fuel consumed for the rich spike is desirably small in amount to minimize the fuel consumption. In the exhaust emission purification device described in the publication cited above, therefore, the amount of fuel consumption for the rich spike is required to be reduced as much as possible.
In view of this situation, the object of this invention is to provide an exhaust emission purification device for reducing the amount of fuel consumption while, at the same time, avoiding sulfur poisoning of a exhaust emission purifier.